Fiberizable glass compositions presently include boron or fluorine containing compounds as fluxing agents, which reduce the viscosity of the batch particularly during the early stages of melting. After recognition of boron and fluorine as potential pollutants, the problem has been to produce a glass composition (1) having the requisite physical properties for fiberization, (2) which is acceptable to the industry, and (3) which does not include fluorine and boron.
For example, E glass, which is the most common glass composition presently used for making textile fibers, has 9 to 11 percent by weight B.sub. 2 O.sub. 3 and may contain fluorine as a fluxing agent. An object of the composition and method disclosed in our copending applications was to provide a substitute for E glass, while eliminating boron and fluorine. The specifications for E glass require that the percentage of alkali metal oxides, namely Na.sub. 2 O, K.sub. 2 O and Li.sub. 2 O, be less than one percent, by weight. Therefore, it is important to maintain the alkali metal oxide level of glass compositions at one percent, or less, when the glass composition is to be used in place of E glass. The composition of E glass is disclosed in U.S. Pat. No. 2,334,961, assigned to the assignee of the instant application. It has now been found that additional amounts of Na.sub. 2 O, K.sub. 2 O and Li.sub. 2 O may be added to the glass composition of this invention without adversely affecting the physical properties necessary for commercial fiberization.
Boron is commonly supplied in the batch composition as colemanite, anhydrous boric acid or boric acid, while fluorine is added as CaF.sub. 2 or sodium silicofluoride (Na.sub. 2 SiF.sub. 6). Melting of the glass batch raw materials is gas-fired furnaces, for example, to form molten glass from which fibers may be drawn and formed includes heating the batch and molten glass to temperatures in excess of 2200.degree. F. Commonly used textile fibers are melted in the range of 2400.degree. to 2750.degree. F. At these melting temperatures, B.sub. 2 O.sub. 3 and F.sub. 2, or various compounds of boron and fluorine, tend to volatilize out of the molten glass and the gases can be drawn up the exhaust stacks and escape into the atmosphere surrounding the glass fiber forming area.
The resultant air and possible water pollution can be reduced or eliminated by a number of approaches. Water scrubbing or filtering of exhaust gases can often clean up exhaust air. Use of electric furnaces in place of gas-fired furnaces will virtually eliminate the losses of volatile fluxes (e.g. boron and fluorine) commonly associated with gas-fired furnaces at temperatures above 2200.degree. F. These clean-up approaches however are often costly and can be avoided if the source of the pollutants can be removed from the glass compositions. Complicating this solution, however, is the fact that removing boron and fluorine removes two commonly used fluxing ingredients in fiberizable, textile glass compositions. Maintaining acceptable melting rates, melting and operating temperatures, liquidus and viscosity in the absence of boron and fluorine has been found to be quite difficult.
An acceptable operating range in a commercial textile glass feeder or bushing is between 2250.degree. and 2500.degree. F. A glass composition that will operate smoothly in this environment preferably should have a liquidus temperature of approximately 2250.degree. F. or less and a viscosity of log 2.5 poises of 2450.degree. F., or less, such that the temperature of a viscosity of log 2.5 poises, less or minus the liquidus temperature (.DELTA.T) is 100.degree. F. or greater.
The temperature at a viscosity of log 2.5 poises is preferably about one hundred degrees Fahrenheit greater than the liquidus temperature to avoid devitrification (crystal growth) in the glass as the fibers are formed. Since devitrification causes irregularities or seeds in the glass, which hamper or may stop fiber production, the liquidus temperature of a commercial textile glass should preferably be less than about 2500.degree. F.
The viscosity of the glass is also a key to efficient and economical fiber forming. Glass viscosities of log 2.50 poises at 2450.degree. F., or more, require such high temperatures to melt the glass and make it flowable and formable into fibers that the metallic bushings or feeders may sag and become unusable or must be replaced or repaired more frequently than bushings contacting less viscous glasses.
It was discovered that the addition of 3 to 6% by weight TiO.sub. 2 to the three phase glass composition, including SiO.sub. 2, Al.sub. 2 O.sub. 3 and CaO, reduced the viscosity of the molten glass to within the fiberization range. The liquidus temperature was still somewhat high for conventional fiberization equipment and techniques, although glass fibers can be successfully made from the four component composition. Another problem with the four component glass composition, in certain applications, was the yellow or brown color of the fibers formed from the molten glass. The color results from the relatively high concentration of TiO.sub. 2 when Fe.sub. 2 O.sub. 3 is present. Iron oxide (Fe.sub. 2 O.sub. 3) is normally present in trace amounts from the raw materials.
The addition of 1.5 to 4% by weight MgO lowered the liquidus temperature within the fiberization range of conventional commercial fiber forming equipment and reduced the required TiO.sub.2 concentration to improve the color of the fibers formed from the glass. The preferred range of TiO.sub. 2 in the five component glass is 3 to 5%, by weight. The color is still somewhat yellow, especially where the TiO.sub. 2 concentration exceeds 4%, and the liquidus temperature and viscosity are still somewhat higher than the preferred range.
It is therefore an object of the present invention to provide a boron and fluorine free glass composition which has the preferred properties of E glass, for example, including color, modulus of elasticity and tensile strength. With these problems in mind, the boron and fluorine free, fiberizable glass compositions and methods of this invention were developed.